The present invention relates to methods of stimulating nerves. More specifically, the present invention relates to determining and setting stimulation parameters, i.e., stimulus pulsewidth and amplitude, for stimulating nerves with electrodes.
When a stimulus pulse is used to stimulate a nerve fiber, there are primarily two stimulation parameters that determine the “capture” (or depolarization) of nerve fiber. These stimulation parameters are the pulse amplitude and the pulsewidth. It is well known that these parameters have an interplay in stimulating a nerve fiber. The values of stimulus pulsewidths and stimulus amplitudes which just achieve capture of nerve fiber (depolarization) are inversely related and can be plotted as an X-Y curve. The resulting stimulation threshold curve is the well-known “strength-duration” curve for an individual nerve fiber. The term “strength” refers to the stimulus amplitude and the term “duration” refers to the pulsewidth.
Clinically, however, it may not always be feasible to detect depolarization of a single nerve fiber. Rather, in a clinical setting, what can be evaluated is the mass depolarization of a set of nerve fibers that make up a portion of a nerve bundle. Because each nerve fiber within a nerve bundle has its own characteristic “strength-duration” curve, stimulating a nerve bundle exhibits a “massed” or composite strength-duration curve of many nerve fibers firing together. Such a composite curve may be derived by stimulating a nerve bundle and measuring a detectable threshold physiological event resulting from nerve stimulation, which threshold event is used as a surrogate for the threshold “capture” of a few nerve fibers in the nerve bundle. In other words, while actual detection of individual nerve fiber depolarization is not easily observed without special detection equipment, the end effect of a few nerve fibers depolarizing can often be reasonably measured by the physiological response to the nerve depolarization.
For example, when sensory nerves that mediate the masking of pain are stimulated, the proxy measure of nerve stimulation threshold can be the resulting, just noticeable perceived, masking of pain (“the perception threshold”) caused by depolarization of some nerve fibers. Other physiological effects of stimulation of nerve fibers may be used as proxies for determining stimulation threshold for other types of applications. For example, when motor nerves are stimulated with a threshold stimulus, a just visible muscle twitch can be used as a visual indicator that some nerves innervating the muscle are being captured.
There are a number of confounding factors in the optimal selection of stimulus pulsewidth and amplitude for nerve stimulation. These factors include inter-patient variability of perception thresholds and maximum tolerable or maximum comfortable thresholds, as well as variability in electrode impedances within a patient and amongst patients.
For nerve stimulation, the usage range (“UR”) of stimulation amplitudes for a constantly held pulsewidth can be defined as the difference between the “maximum comfortable threshold” and the “perception threshold.” “Maximum comfortable threshold” is a stimulus level (pulsewidth, amplitude) where beyond that level a patient perceives discomfort or even pain. The “perception threshold” is that stimulus level (pulsewidth, amplitude) where the patient just notices a physiological effect. For example, in cochlear application, in may be a perception of sound. In spinal cord application, it may be a tingling sensation. When narrower pulsewidths are used, the UR of stimulation amplitudes shifts and increases. The UR is expanded because narrower pulsewidths must be compensated by using larger stimulus amplitudes and, hence, each discrete step of programming amplitude provides lesser effect.
Most commercial stimulation systems provide discrete programmable steps of stimulation amplitudes as well as steps of pulsewidths. If a large pulsewidth, e.g., greater than 200 microseconds is selected, the discrete amplitude steps of a stimulator may provide too great an effect between one amplitude step and the next step. Thus, a pulsewidth should be chosen to permit the use of programmable amplitude steps which can finely control the delivery of stimulus energy. This control is achieved when the UR is large, such that each discrete, programmable amplitude step provides a significant, measured increase in stimulation to target nerve fibers and there are a reasonable number of steps between perception and maximum comfortable thresholds.
A related concern is that one cannot simply choose the minimum duration pulsewidth merely to achieve the largest UR. In practice, a stimulator system does not have an infinite compliance voltage. This system compliance voltage limits the absolute stimulus amplitude that can be delivered through the electrodes. High electrode impedance may accentuate this compliance voltage limiting effect. Using a very narrow pulsewidth requires a large compensating stimulus amplitude to capture a target nerve fiber. Such a required stimulus amplitude, however, can exceed the maximum deliverable amplitude of a stimulator which is determined by the system compliance voltage. Thus, in practice, selecting a pulsewidth which is too narrow will prevent the capture of a nerve regardless of the size of the stimulus amplitude chosen.
Sometimes a fixed pulsewidth, such as 210 microseconds, is chosen based on past experience. Using a fixed, relatively large, nominal pulsewidth, such as 210 microseconds, also does not provide optimal stimulation because electrode positions relative to the target nerve fibers can vary widely between patients and within a patient. If such a fixed pulsewidth is used, it may be difficult to differentiate the perception threshold and maximum comfortable threshold. The UR may be too small which can make fine stimulation control difficult. In sum, because there is wide variability between patients and electrode placement/configuration, there is no single pulsewidth value that suffices for every occasion.
To determine the optimum stimulus, many random, stimulus parameter sets (pulsewidth, amplitude) may be delivered through the electrode and the result of each stimulus may be evaluated until a good parameter set is found. However, such a random method can be inordinately time-consuming and taxing to a patient as the number of possible combinations is large.
One example application where an optimal stimulus pulsewidth and amplitude must be determined is in spinal cord stimulation (SCS). SCS systems typically include an Implantable Pulse Generator (IPG) coupled to an array of electrode contacts at or near the distal end of a lead. The IPG generates electrical pulses that are delivered to neural tissue, e.g., the dorsal column fibers within the spinal cord, through the contacts of the electrode array. The electrode contacts can be implanted proximal to the dura mater of the spinal cord. Individual electrode contacts, which may be loosely referred to as “electrodes”, can be arranged in a desired pattern on a lead in order to create an electrode array. Individual conductor wires or leads can connect with each electrode contact in the array. The lead may exit the spinal column and attach to the IPG, either directly or through one or more lead extensions.
In an SCS system for treating chronic pain, the electrical stimulus pulses delivered by the system typically have the effect of producing a tingling sensation known as a paresthesia. The paresthesia helps block or at least masks the chronic pain felt by a patient. The stimulus amplitude and stimulus pulsewidth affect the intensity and location of the paresthesia felt by the patient. In general, it is desirable to have the stimulus amplitude and pulsewidth comfortably set to a level which produces paresthesia to block pain but not above a level that may actually result in discomfort or pain apart from the native pain. Moreover, the stimulus amplitude and pulsewidth should be set to a level lower than that which can recruit reflex motor nerves that can cause involuntary muscle contractions.
SCS and other stimulation systems are well accepted for treating chronic pain. An implantable electronic stimulator is disclosed, for example, in U.S. Pat. No. 3,646,940 that provides timed, sequenced electrical impulses to a plurality of electrodes, which patent is herein incorporated by reference. As another example, U.S. Pat. No. 3,724,467, teaches an electrode implant for neurostimulation of the spinal cord, which patent is herein incorporated by reference. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted, and which is controlled by an external controller.
Accordingly, what is needed is a method for efficiently selecting optimal stimulus pulsewidth and amplitude, particularly for the SCS application, which provides the best overall UR and stimulation efficacy for a given patient and electrode configuration.